Add preliminary source mapping to parser (always on)

unittests/asm_test.c

@@ -31,6 +31,7 @@ int main() {
     }
     assert(pres.status == DST_PARSE_OK);
     dst_puts(dst_formatc("\nparse result: %v\n\n", pres.result.value));
+    dst_puts(dst_formatc("\nparse map result: %v\n\n", pres.map));
 
     /*
     opts.flags = 0;

unittests/nanbox_test.c

@@ -33,12 +33,20 @@ union dst_t {
  * 47 bit payload representaion is that the type bits are no long contiguous. Type
  * checking can still be fast, but typewise polymorphism takes a bit longer. However, 
  * hopefully we can avoid some annoying problems that occur when trying to use 47 bit pointers
- * in a 48 bit address space (Linux on ARM) */
+ * in a 48 bit address space (Linux on ARM), or when generating Signaling NaNs. */
 
 /*                    |.......Tag.......|.......................Payload..................| */
 /* Non-double:        t|11111111111|1ttt|xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */
 /* Types of NIL, TRUE, and FALSE must have payload set to all 1s. */
 
+/* Other possible representations: */
+
+/* Common Style */
+/* Non-double:        1|11111111111|tttt|xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */
+
+/* LuaJIT style */
+/* Non-double:        1|11111111111|1ttt|txxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */
+
 /* Double (no NaNs):   x xxxxxxxxxxx xxxx xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */
 
 /* A simple scheme for nan boxed values */